1. Field of Invention
This invention relates to the field of x-ray fluorescence inspection apparatuses and in particular to portable x-ray fluorescence spectrometers.
2. Description of Related Art
Heretofore various x-ray fluorescence spectrometers have been proposed and developed for elemental analysis. In these devices, the sample has been excited by employing either a radioactive isotope or an x-ray tube. Obsolescence of the use of a radioactive isotope is discussed in U.S. Pat. No. 5,528,647 and in which a method for using an x-ray tube in conjunction with a filtering mechanism is described. Due to increasing environmental and governmental requirements and regulations, the use of radioactive isotopes for excitation purposes has not been favored in the design of these devices. With respect to portable x-ray spectrometers, the need for a lightweight, and a low cost unit have forced the designer to continue to employ radioactive isotopes as the excitation source. This has the disadvantage of both increased environmental and safety compliance issues associated with the radioactive source as well as the disadvantage of the lack of control over the excitation characteristics of the source. The inability to control the excitation parameters of the radioactive source is a hindrance for the use of the apparatus for certain applications where excitation selectivity is required. More recent approaches to this limitation have resulted in apparatus that no longer contains a radioactive isotope as the excitation source but employs a thermionic x-ray tube. The x-ray tube is powered by an on-board high voltage supply as well as a filament supply necessary to heat the thermionic cathode. These components are separate components linked by a high voltage cable, typically operating in a manner such that heat dissipation from the isolated anode limits extended use of the device. This also results in a unit which is much larger, heavier and more costly than portable x-ray fluorescence spectrometers which rely on radioactive isotopes for excitation. Predominately, these portable devices are limited for single purpose use, such as transition element identification and quantification for the purposes of alloy identification.
Accordingly, several objects and advantages of the invention are: (1) The use of an x-ray tube coupled with a high voltage power supply in a single lightweight integrated component. This eliminates the need for a radioisotope as the excitation source as well as mitigates the disadvantages of prior systems which employed x-ray tubes and high voltage power supplies as separate components resulting in a device considered too heavy for portable x-ray spectroscopy. (2) The use of a cold cathode x-ray tube for portable x-ray spectroscopy. This allows for the elimination of the thermionic cathode and its associated power supply, thus, reducing the amount of power, weight and heat required as compared to previously portable x-ray spectrometers. (3) The use of an integrated emission extraction and/or suppression grid placed between the non-thermionic cathode and the target anode. This allows for the accurate control of both emission current as well as introduces high emission current stability through the integrated feed back loop between the control grid voltage and the total electron beam emission current. (4) The use of compact, lightweight SMT electronic boards in conjunction with digital signal processing micro controllers. This allows for complex spectral processing on the portable device providing the possibility of addressing multiple applications rather than previous application specific devices. Furthermore, through the use of multiple layer surface mount electronic boards, the overall volume requirement is reduced resulting in a smaller and lighter device than previous devices. (5) The use of an integrated embedded personal computer and associated color LCD display in conjunction with a graphical user interface. This allows for comprehensive on-board software to address multiple tasks, such as qualitative analysis, quantitative analysis, alloy identification, unknown analysis, presented in a familiar Microsoft Windows CE operating system. This integration further mitigates the limitations imposed by smaller; memory limited micro controllers, allowing for a comprehensive x-ray spectroscopy system in a portable device. (6). The use of an ergonomic design which balances the center of gravity in a neutral position on the wrist. This allows the user to see the display during analysis while allowing for extended operation of the device in an ergonomically correct orientation. (7) The use of a docking station coupled with a desktop personal computer in conjunction with a graphical user software system designed to perform extended operations not practical in a portable x-ray spectrometer. This allows for multiple applications to be developed on a single device, such as alloy identification, wood preservative analysis, plastic filler analysis, forensic analysis applications, and other applications suitable for x-ray fluorescence spectroscopy. This object also allows for comprehensive results storage and reporting as well as the control of resident applications on the portable x-ray spectrometer. This objective also allows for the operation of the device is a desktop laboratory fashion. In this manner a sample for analysis may be brought to the spectrometer for analysis rather than the portable spectrometer needing to be position on the sample for analysis. In this manner, the device can be operated as a laboratory grade spectrometer for new method development as well as for those applications where sample stability and precision are at issue. Further objects and advantages of this invention will become apparent from a consideration of the drawings and ensuing description.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentality""s and combinations particularly pointed out in the appended claims.
To mitigate such limitations a portable x-ray spectrometer is disclosed, which comprises an x-ray tube based upon a non-thermionic cathode in conjunction with the use of energy filtration, supplied as a primary beam filter placed between the x-ray tube output and the sample to be analyzed. The apparatus uses a novel approach to the required high voltage electronics to reduce the size and weight of the overall device by integrating the x-ray tube and high voltage power supply into a single lightweight, low volume, component. Furthermore, as the x-ray tube employs a cold-cathode design, no x-ray filament power supply is required, thereby eliminating weight, heat, and cost. Selective excitation of the sample is determined by a control grid integral to the x-ray tube and controlled by the integrated power supply through a feed back loop. Further refinement of the primary excitation beam is provided through the use of a multi position primary beam filter.
In designing a portable x-ray spectrometer for applications such as alloy identification, size, weight and cost play an important role. To achieve all three of these constraints, novel technology must be employed in concert. This involves eliminating sources of heat and weight while focusing on reduced manufacturing costs. This leads to a design, which integrates multiple surface mount technology (SMT) electronic boards to minimize size and power consumption. As such, the invention employs SMT electronic boards coupled with programmable digital signal processing (DSP) for the complex sorting of energy specific pulses from the x-ray detector. The overall size of the multi channel analyzer (MCA) and analog processing control board is kept to below 2.5 square inches. Further emphasis is placed on the integration of the x-ray tube and the high voltage power supply to minimize weight whilst providing required high voltage standoff of a 35 kV x-ray tube based system. The invention incorporates a grounded target design to maximize heat dissipation from the x-ray tube anode to the case. This results in the need for a high voltage power supply operated in a negative mode with an isolated control grid placed between the non-thermionic cathode and the target anode. The overall size of the high voltage power supply and isolated control grid power supply are kept to a minimum through SMT resulting in a combined supply of 4.75 cubic inches total volume.
As the present invention is intended for use by non-skilled workers, the user interface plays a critical role. As such, the integration of an embedded personal computer coupled with a high-resolution color liquid crystal display is employed. In conjunction the invention integrates this hardware with Microsoft Windows CE operating system platform to provide the user with an integrated graphical user interface for operation of the device. Furthermore, the invention provides for the integration of the portable x-ray spectrometer to a desktop personal computer and associated graphical user interface by the means of a docking station. This docking station consists of a mechanism for operation of the device as a bench top/laboratory based spectrometer, as well as integration with the desktop personal computer. In this manner, the user is able to perform more complex tasks not available on the portable device itself, such as results storage and management, results reporting, application development and method setup. The invention allows for new analytical method development to be performed in conjunction with the desktop personal computer and associated software and for the download of necessary operational information to the portable x-ray spectrometer. In this manner, the user is able to perform multiple analysis of different materials, bound by limitations of the x-ray fluorescence technique and not by limitations imposed by the on board computer memory.